Mechanisms underlying intestinal absorption of lipids will be studied in the rat. The lipids of interest are fatty acids, cholesterol, phospholipids and plant sterols. These compounds interact with one another and with the bile acids in the intestinal lumen to form molecular aggregates or micelles, a key step in the luminal phase of lipid absorption. It is the purpose of the project to study the mutual interaction of these compounds and their mechanisms of uptake by the epithelial cells. The rate limiting step in the absorption of lipids has been suggested to be diffusion across an unstirred water layer overlying the epithelium. Presumably the micelle exists to improve the efficiency of absorption by partially overcoming this barrier. It may also exist in order to facilitate the transfer of the lipids across the epithelial membrane. This idea will be tested in the present study. The study will also include an examination of the bile acids with the view to determining their full spectrum of actions.